Scanning MEMS mirrors are used in numerous applications, such as e.g. video projectors, optical switches for communication networks, laser printers, laser scanners or LiDAR systems. The expression “LiDAR” can be regarded as an artificial word composed of the expressions “light” and “radar”. LiDAR methods can be regarded as monitoring methods which measure a distance to a target by illuminating the target with pulsed laser light and detecting the reflected pulses using a sensor. In this case, laser light is directed onto a mirror that is caused to oscillate using a mirror drive and deflects the light beam toward the left and toward the right. In this way, a region in front of the system is scanned by the laser. The angle between the outermost left and the outermost right point of the region is referred to as the field of view. The angle is double the magnitude of the maximum deflection angle of the mirror. If a mirror oscillates between −10° and +10°, for example, the laser is deflected between −20° and +20° and the field of view is 40°. It is desirable to ensure a specific field of view. The field of view is defined by the amplitude of the oscillation, e.g. the maximum inclination angle of the oscillation. Therefore, the mirror drive should have a unit for detecting the mirror inclination angle and for regulating the maximum inclination angle or the amplitude of the mirror oscillations on the basis thereof. By way of example, a specific application may have a field of view of 40°. The objective of amplitude regulation is then to ensure that the mirror is inclined in such a way that this field of view is attained with a sufficient accuracy.
Various methods have been implemented for driving such mirrors and detecting the position of the mirror. One such method is a capacitive drive. In the case of a capacitive drive, the mirror is driven by electrostatic forces that pull the mirror. A set of fingers connected to the mirror (the rotor) overlaps a set of static fingers on a frame, the stator. Since the sets of fingers have the forms of combs, this type of drive is also referred to as a comb drive.
When the mirror moves in from its highest inclination angle (in the direction of its rest position), a constant voltage is applied between the rotor and the stator, such that the rotor is attracted by the stator. When the mirror oscillates through its zero position, the voltage is switched off and the mirror oscillates freely out into the opposite position. When the mirror begins to oscillate back, the voltage is switched on again. The highest oscillation amplitude can be attained if the frequency of the actuation voltage is equal to double the resonant frequency of the mirror. However, this type of resonance operation is not a prerequisite.